Wednesday, October 28, 2009

Scientists Discover Magnets That Have Only One Pole

Scientists have long speculated about the existence of magnetic materials that have only one pole, instead of both negative and postive poles. Even our resident mad scientist has written about the subject. But now, Claudio Castelnovo of the University of Oxford thinks that his team has found evidence of the existence of these monopoles in certain rare-materials known as “spin ices”:

At low temperatures, there is still some magnetic wiggle room in the spin ice’s lattice structure, but not a lot—the magnetic freedom of the system is frustrated, so to speak. “As a result, this is a substance that has degrees of freedom that look the same, microscopically, as you would see in afridge magnet,” Castelnovo says. “But a fridge magnet is able to order so as to act as a fridge magnet and stick to metals, while this one is not able to achieve this level of ordering in spite of having this magnetic structure inside, because of this frustration.”

Internally, the tiny magnetic components arrange themselves head to tail in strings, like chains of bar magnets stretching across a table in different directions. In a very cold, clean sample, those strings form closed loops. But excitation induced by a rise in temperature can introduce tiny defects inthese chains, Castelnovo says—in the bar-magnet analogue, one of the magnets is flipped, breaking the head-to-tail continuity. “You have your path that is north–south–north–south, and at a certain point one of the needles actually twists 180 degrees and points the wrong way,” he explains.
On either side of that defect, all of a sudden, is a concentration of magnetic charge—two norths at one end, two souths at the other. Those concentrations can float free along the string, acting as—voilĂ —magnetic monopoles.

magnetic_field t123

Have we reached magnetic polarity escape velocity yet?

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